Generally speaking, the present invention relates to antennas for portable communication apparatuses, such as mobile telephones. More specifically, the invention relates to an antenna of the type comprising a radiator having a first end for connection to radio circuitry in the portable communication apparatus, and a second end.
A portable communication apparatus, such as a mobile telephone, a cordless telephone, a portable digital assistant, a communicator or a paging device, requires some form of antenna in order to establish and maintain a wireless radiolink to another unit in a telecommunication system. A widely used antenna in this field is a stub or helix antenna, comprising a helically wound thin metal wire or ribbon, which is embedded in a protective molding of dielectric material, or is alternatively covered by a dielectric radome. FIG. 24 illustrates a schematic mobile telephone 1 having such a stub or helix antenna 2 mounted on the exterior of a top surface of the apparatus housing of the mobile telephone.
FIG. 1 provides a schematic illustration of a miniaturized end-fed halfwave helix antenna according to the prior art. The antenna comprises a helical radiator 10 having a first end 11, to which an impedance matching circuit 13 is connected. The purpose of the impedance matching circuit 13 is to match the high input impedance (for instance about 200 ohm) of the end-fed halfwave helical radiator 10 to the lower impedance (normally 50 ohm) of a coaxial connector or coaxial cable, which in turn is coupled to radio circuitry within the portable communication apparatus. The helical radiator 10 has a free second end 12. When fed with an electric signal at appropriate frequency(-ies) from the radio circuitry of the portable communication apparatus through the impedance matching circuit 13, the helical radiator 10 acts as a halfwave dipole antenna, as is schematically illustrated by a current arrow I in FIG. 1.